Mass Analyzers for ICP-MS

Based on a quadrupole ICP-MS, another technique, termed collision/reaction cell, has frequently been used to reduce polyatomic ion interferences. These techniques provide simple, efficient, and low-cost methods in the face of many difficult interference problems. In these methods, ions to be analyzed first enter a radio-frequency-only multipole (e.g. a quadrupole, hexapole, or octapole), in which the analytes react with the collision/reaction gas, which is usually oxygen, ammonia, xenon, or methane, to remove polyatomic interference or generate a new analyte ion of mjz showing less interference. The RP-only multipole does not separate ions like a traditional quadrupole, but it has profound influence on collisional focusing of ions, both of the energy and spatial distributions. An example of removing the polyatomic interference is shown below, which uses ammonia gas to reduce any Ar interference in the measurement of Ca  

Because of the disparity of the reaction rates of the two neutralization reactions, the analyte can be efficiently determined after the introduction of ammonia as a reactive gas into the multipole. There are many excellent reviews about the development and applications of collision/reaction cell in ICP-MS. In order to eliminate the new isobaric interferences produced by secondary reactions, two methods are commonly used in the commercial instrument the discrimination of kinetic energy or mass filtering. The former mainly utilizes the post-cell kinetic energy discrimination (KED) to suppress transport of the produces of the side reactions to the analyte in the hexapole and octapole cell instruments. Whereas in the latter, the quadrupole cell has a capability to reduce the formation of the unwanted side product ions by selecting an appropriate mass bandpass. The details of the KED and bandpass approaches can refer to many excellent books and reviews.  

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The most popular mass analyzers for ICP-MS have been quadrapole, magnetic-sector, and double-focusing analyzers, although time-of-flight analyzers are also used. These analyzers vary in resolution, throughput, and scanning time. The resolution of a mass analyzer is defined as ... 

Most mass analyzers for ICP/MS instruments are operated in a scanning mode. These mass analyzers operate as bandpass filters, passing a single mass-to-charge ratio at a time for detection by the ion collection system. The mass analyzers are scanned either by peak-hopping or by continuous scan. 

If a sample solution is introduced into the center of the plasma, the constituent molecules are bombarded by the energetic atoms, ions, electrons, and even photons from the plasma itself. Under these vigorous conditions, sample molecules are both ionized and fragmented repeatedly until only their constituent elemental atoms or ions survive. The ions are drawn off into a mass analyzer for measurement of abundances and mJz values. Plasma torches provide a powerful method for introducing and ionizing a wide range of sample types into a mass spectrometer (inductively coupled plasma mass spectrometry, ICP/MS). 

Pd removal was determined as follows. An aliquot of a representative liquid or solid sample was accurately weighed and subsequently digested by refluxing in nitric and/or hydrochloric acid using a closed vessel microwave procedure (CEM MARS5 Xpress or Milestone Ethos EZ). Cooled, digested samples were diluted, matrix matched to standards, and referenced to a linear calibration curve for quantitation an internal standard was employed to improve quantitation. All samples were analyzed by an Inductively Coupled Plasma Mass Spectrometer or ICP/MS (Perkin Elmer SCIEX Elan DRCII) operated in the standard mode. 

An advantage of ICP-MS compared to all other atomic mass spectrometric techniques including TIMS is that usually only simple sample preparation (e.g., by microwave induced digestion of solid samples) is necessary. Sample preparation steps for ICP-MS analyses are similar to those of ICP-OES. Concentrated solutions are analyzed after dilution with high purity water only. In order to correct mass drifts of the instrument, an internal standard element like In or Ir with known concentration (e.g., I Op.g 1) is added. The solution is then acidified with HN03 to stabilize the metal ions in aqueous solution. 

A 9 mL aliquot from each TIMS sample solution was submitted to the University of Georgia, Laboratory for Environmental Analysis, for inductively coupled plasma-mass spectrometry analysis (ICP-MS). A Perkin-Elmer Elan 6000 ICP-MS with quadrapole chamber mass detector system was used to analyze the solution for Ag, As, Cu, Sb, Sn, Pb, and Zn. Insufficient sample remained for further analysis or replicate samples. However, all appropriate blanks, dilutions, and standards were run. 

Trace elemental analysis of ancient ceramics has been proven a very useful tool for tracing the circulation of this material. Instrumental neutron activation analysis (INAA) was for years the analytical technique of choice to measure the composition of ceramics because of the large number of elements it could determine and its good sensitivity. Lately, a few publications have shown that laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) could provide similar results as INAA more quickly and at lower cost. A protocol has been developed to determine 51 elements using LA-ICP-MS and tested it on Wari period ceramics previously analyzed using INAA. We show how INAA and LA-ICP-MS analysis lead to the same conclusion in terms of sample groupings. 

Arsenic also has been accurately analyzed by ICP/MS. The specimen is prepared in dilute acid containing gallium as an internal standard and aspirated directly into the argon plasma. Mass response from the argon plasma is monitored for As (75m/z), gaUium (70m/z), and (51m/z) to... 

Once the methodology for trace element determination in petroleum products by ICP-MS was validated, it was applied to several crude oils (supplied by Petrobris) and their fractions originating from a Brazilian petroleum basin. In order to prove the reliability of analytical results obtained after oil fractionation, total mass balance (TMB) was calculated for each element measured. Table 3 shows an example of crude oil and its fractions analyzed by ICP-MS. TMB results were in the range of 90-110 % for most of the elements analyzed. In order to confirm the repeatability of oil fractionation and analysis, each sample (oil + fractions) was analyzed three... 

Most elements are efficiently ionized at the high temperature of the plasma. For most metals (ionization potential <8 eV), ionization is typically greater than 90%. Many elements have an ionization efficiency >98%. The few elements not analyzed by ICP/MS are H, He, C, N, O, F, Pm, Ne, Ar, Kr, and short-lived radionuclides. These exceptions have high ionization potentials, severe spectral interference, or are better measured by radiometric counting or other mass spectrometric methods. 

Mass analysis is simply a method of separating ions of different mass-to-charge ratio [mlz). However, since the ions of interest are almost exclusively singly charged the mlz is equivalent to mass for most practical purposes. There are three types of mass analyzer used for ICP-MS, quadrupole, time-of-flight (TOF), and magnetic sector. 

The main advantage of magnetic sector analyzers is their vastly superior resolution compared with quadrupole instruments. The resolution of a mass analyzer can be expressed as (R = M/AM), where R is the resolution, M is the mass of the isotope of interest, and AM is the peak width of the isotope at 5% peak height. The increased resolution is advantageous because polyatomic ion interferences can be resolved from analyte isotopes of interest. Table 1 shows a number of polyatomic ion interferences and the resolution required to separate them from the elemental isotope of interest. Most quadrupole and TOF mass analyzers operate with an upper resolution of 400, which enables unit mass resolution, while magnetic sector instruments for ICP-MS have been operated up to a resolution of 10 000 enabling peaks of fractions of a mass unit to be resolved. 

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